1. Field of the Invention
The present invention relates to a compound semiconductor device, and more particularly, to a resonant tunneling semiconductor device having a negative differential resistance characteristic caused by resonant tunneling phenomenon.
2. Description of the Related Art
The resonant tunneling semiconductor device uses a resonant tunneling barrier structure, i.e., a quantum-well structure, comprising two barrier layers and a well layer sandwiched therebetween. The resonant tunneling of a carrier (i.e., electron) occurring in the structure provides the negative resistance characteristic.
Resonant tunneling semiconductor devices such as a resonant tunneling barrier (RTB) diode, a resonant tunneling bipolar transistor (RBT) and a resonant tunneling hot electron transistor (RHET), have been proposed. For example, EP No.-017734-A2 (corresponding to U.S. Ser. No. 754,416) discloses a type of RHET. FIG. 1 shows a known RTB diode. This RTB diode comprises a substrate 1, a first contact layer 2, a first barrier layer 3, a well layer 4, a second barrier layer 5, a second contact layer 6, a protective insulating layer 7, a first ohmic-contact electrode 8, and a second ohmic-contact electrode layer 9.
The data of the above-mentioned parts of the RTB diode is as follows, by way of example.
(a) Substrate 1
Material: semi-insulating GaAs PA1 Material: n-type GaAs PA1 Impurity concentration: 1.times.10.sup.18 cm.sup.-3 PA1 Thickness: 0.5 .mu.m PA1 Material: i-type Al.sub.0.33 Ga.sub.0.67 As PA1 Thickness: 50 .ANG. (5.0 nm) PA1 Material: i-type GaAs PA1 Thickness: 56 .ANG. (5.6 nm) PA1 Material: i-type Al.sub.0.33 Ga.sub.0.67 As PA1 Thickness: 50 .ANG. (5.0 nm) PA1 Material: n-type GaAs PA1 Impurity concentration: 1.times.10.sup.18 cm.sup.-3 PA1 Thickness: 0.3 .mu.m PA1 Material: Silicon dioxide (SiO.sub.2) PA1 Thickness: 0.3 .mu.m
(b) First contact layer 2
(c) First barrier layer 3
(d) Well layer 4
(e) Second barrier layer 5
(f) Second contact layer 6
(g) Protective insulating layer 7
In the RTB diode, the first and second barrier layers 3 and 5 and the well layer 4 having a thickness of less than the de Broglic wavelength of the carrier (i.e., electron) form a quantum-well structure.
FIG. 2 shows a conduction energy-band diagram of the RTB diode, for explaining the operation thereof when a bias voltage is applied to the RTB diode. In the drawing, "E.sub.c " and "E.sub.0 " indicate the bottom of the conduction band and the first resonant level for the electron in the well layer 4, respectively. Where the first contact layer 2 applies a positive potential to the second contact layer 6, to correspond the bottom E.sub.c of the conduction band of the second contact layer 6 with the first resonant level E.sub.0 in the well layer 4, as shown in FIG. 2, namely, where the voltage V between the first and second contact layers 2 and 6 becomes: EQU V.apprxeq.2E.sub.0 /q
wherein q is the electric charge of the electron, electrons tunnel from the second contact layer 6 to the first contact layer 2 through the barriers due to the resonant tunneling effect, and thus a current flows in the RTB diode. On the other hand, where the voltage V becomes V&gt;2E.sub.0 /q or V&lt;2E.sub.0 /q, such the voltage conditions are different to the resonant tunneling condition, the amount of electrons tunneling into the first contact layer 2 is remarkably decreased, and thus very little current flows.
FIG. 3 show a current-voltage characteristic of the RTB diode, in which the abscissa axis and the ordinate axis represent the voltage and the current density, respectively. In the drawing, "V.sub.p " indicate a voltage at which a maximum current (peak current) flows due to the occurrence of resonant tunneling, "V.sub.v " indicates a voltage at which a minimum current (valley current) flows when resonant tunneling does not occur, J.sub.p indicates a maximum current density, and J.sub.v indicates a minimum current density.
As can be seen from FIG. 3, the current first increases as a voltage increases. When the voltage becomes V.sub.p, the current reaches the maximum current density J.sub.p, and then the current sharply decreases. When the voltage becomes V.sub.v, the current reaches the minimum current density J.sub.v, and thereafter, the current increases again. Namely, where the voltage varies from V.sub.p to V.sub.v, the current density decreases from J.sub.p to J.sub.v for the voltage increase, and thus a negative resistance characteristic appears.
FIG. 4 shows a current-voltage characteristic of a conventional RTB diode having a diameter of 50 .mu.m, which was obtained by measurement under a temperature of 77.degree. K. In the drawing, the abscissa axis represents applied voltage and each graduation on the scale thereof represents 200 mV. The ordinate axis represents current and each graduation on the scale thereof represents 50 mA. In this case, as can be seen from FIG. 4, the applied voltage corresponding to the minimum current I.sub.v is shifted to a lower voltage side with respect to the voltage corresponding to the maximum current I.sub.p, which occurs because of variations of the amount of voltage drop due to a series resistance component.
The above-mentioned negative differential resistance characteristics in the RTB diode can be also obtained in an RHET and can be used in the constitution of logic circuits, frequency multipliers, oscillation circuits, etc., to reduce the number of elements required for those functions. In such cases, in order to operate the circuits at a high-speed and with a large noise margin, the ratio of the maximum current density to the minimum current density (peak-to valley current density ratio, J.sub.p /J.sub.v) must be large and the maximum (peak) current density J.sub.p must be large. The negative differential resistance characteristics including the J.sub.p /J.sub.v ratio and the J.sub.p value can vary in accordance with a barrier height of the quantum-well structure, a width (thickness) of the barrier layers, a width (thickness) of a well layer, an impurity concentration of the contact layers lying outside the barrier layers, and the like. Conventional AlGaAs/GaAs RTB diodes have maximum (peak) current density J.sub.p values and peak-to-valley current density (J.sub.p /J.sub.v) ratios, as shown in FIG. 5. As can be seen from the drawing, as the maximum current density J.sub.p increases, the J.sub.p /J.sub.v ratio decreases. For example, when the J.sub.p value is approximately equal to 2.times.10.sup.4 A/cm.sup.2, the J.sub.p /J.sub.v ratio is approximately equal to 3. Thus, in the conventional negative resistant element of an AlGaAs/GaAs system, where the J.sub.p value is made larger, the J.sub.p /J.sub.v ratio decreases.